The present invention relates to a nuclear magnetic resonance spectrometry method and, more particularly, to a nuclear magnetic resonance spectrometry method for observing quaternary carbons.
When the structure of an organic compound is analyzed by obtaining a .sup.13 C NMR spectrum from it, the determination of the number of hydrogen atoms coupled to each carbon and the subsequent analysis of the chemical shifts of the carbons which are caused by the hydrogens must always be performed. In .sup.13 C NMR spectra where carbons remain coupled to hydrogen nuclei, a line due to methyl group (CH.sub.3) splits into a quartet (FIG. 1(a)), a line due to methylene group (CH.sub.2) splits into a triplet (FIG. 1(b)), and a line due to methine group (CH) splits into a doublet (FIG. 1(c)). Quaternary carbons (cation linked to four other C atoms) which are not coupled to any hydrogen nucleus appear as a singlet. Thus, it is possible to determine from this manner of splitting how many hydrogens are coupled to each carbon. However, in case where a singlet .alpha. of quaternary carbons is superimposed on a quartet, a triplet, or a doublet which is attributable to CH.sub.3, CH.sub.2, or CH, as shown in FIG. 1(d), it is likely that the peaks of the quaternary carbons are overlooked.
A method has been proposed which extracts and observes only the subspectra of quaternary carbons. Referring to FIG. 2a and FIG. 2b, the timings at which RF pulses are applied to carbon nuclei as well as the timing at which the resulting signal is observed are shown in FIG. 2a. The timing at which a decoupling RF pulse is applied to hydrogen nuclei is shown in FIG. 2b. The resulting free induction decay signal, or an echo signal, is sampled for a period of F. The signal is noise-modulated for the period indicated by the hatching. This pulse sequence removes all the peaks of CH, CH.sub.2, CH.sub.3, etc. other than the peaks of quaternary carbons from the spectrum. This technique is described, for example, in J. Chem. Soc., Chem. Commun., 1982, pp. 1138-1140.
According to this prior art pulse sequence, hydrogen nuclei are fully decoupled. Therefore, fine splitting of the peaks of quaternary carbons which would otherwise be caused by the long-range, weak coupling between the quaternary carbons and the adjacent hydrogen nuclei cannot be observed. As a result, it is utterly impossible to know how many hydrogen atoms exist near the quaternary carbons.